Making solar cells practical as energy sources has been promoted. Various types of solar cells have been developed. Known typical solar cells include silicon-based solar cells, inorganic compound-based solar cells, organic solar cells, and other solar cells.
A solar cell module has a front surface protection sheet disposed on the side on which sunlight is incident, to protect the surface. The solar cell module also has a solar cell rear surface protection sheet (solar cell back sheet) disposed on the opposite side to protect power generation cells. Such solar cell rear surface protection sheets are required to have weatherability, water resistance, heat resistance, moisture proof properties, gas barrier properties, and like properties to minimize deterioration in the long-term performance of solar cell modules.
To obtain solar cell rear surface protection sheets with the above properties, various films have hitherto been used. Examples of such films include metallic foils, metal plates, and metal vapor deposited films, such as aluminum, copper, and steel plates; films of plastics, such as polypropylene, polyvinyl chloride, polyesters, fluororesins, and acrylic resins; and the like. Laminates of these films are also used to further improve performance.
FIG. 1 illustrates an example of a laminate comprising films. A solar cell rear surface protection sheet 10 is a laminate of a plurality of films 11 and 12, and the films 11 and 12 are laminated together by means of an adhesive 13. A typical method for laminating films is dry lamination, and the adhesive 13 is required to have sufficient adhesion to the films 11 and 12.
FIG. 2 illustrates an example of a solar cell module. A solar cell module 1 comprises the solar cell rear surface protection sheet 10 in combination with a sealing member 20 for sealing power generation cells, power generation cells 30, a glass plate 40, and a frame 50.
The solar cell module 1, which is exposed to an outdoor environment over a long period of time, is required to have sufficient durability with regard to high temperature, humidity, and sunlight. In FIG. 1, particularly if the adhesive 13 were to have poor performance, the films 11 and 12 would peel off, thus impairing the appearance of the solar cell rear surface protection sheet 10 over time. Accordingly, the adhesive for solar cell rear surface protection sheets is required to be sufficient to prevent the films from being peeled off from each other even when the solar cell module 1 is exposed to an outdoor environment over a long period of time.
An example of such an adhesive for solar cell rear surface protection sheets is a urethane adhesive. Patent Literature 1 to 3 disclose adhesives for solar cell rear surface protection sheets, the adhesives containing a curing agent such as an isocyanate in combination with a polyol to improve durability and hydrolysis resistance.
Patent Literature 1 discloses, in the Examples, that a solar cell rear surface protection sheet was produced using a urethane-based adhesive for lamination. Patent Literature 2 and 3 disclose that an isocyanate curing agent was mixed with an acrylic polyol to produce an adhesive (see Tables 1 and 2 of Patent Literature 2 and Tables 1 and 2 of Patent Literature 3), and that a solar cell rear surface protection sheet with excellent long-term weatherability and hydrolysis resistance was produced using the adhesive.
Patent Literature 1 to 3 teach that a solar cell rear surface protection sheet prepared using an adhesive with excellent hydrolysis resistance and lamination strength allows for the prevention of deterioration in the appearance of a solar cell module. However, the types of films for use in solar cell rear surface protection sheets tend to increase year by year, and such films are designed to have high hydrophobicity to improve the hydrolysis resistance. Examples of materials with excellent hydrolysis resistance include linear low-density polyethylene (LLDPE), polyester films, fluorine-containing films, inorganic vapor deposited films, and the like. However, LLDPE, polyester films, fluorine-containing films, inorganic vapor deposited films, etc., are materials with poor adhesion. Thus, an inability to provide sufficient adhesion strength and the problem of sudden peeling, called zipping, are likely to occur. Additionally, since the weatherability of the films themselves is improving and higher performance is required for solar cell modules year by year, the performance required for adhesives for use in solar cell rear surface protection sheets is also inevitably increasing. Therefore, it can hardly be said that the solar cell rear surface protection sheets of Patent Literature 1 to 3 sufficiently satisfy consumer demand.